Overshadowed by the Shard, Guy’s Hospital towers in south London are undergoing a major transformation. Mark Hansford reports.
While construction of Europe’s tallest building, the Shard at London Bridge, has been grabbing headlines with its sheer scale, two towers living in its shadow are becoming noteworthy for engineering of a more intricate nature.
Just across the road from the Shard, Balfour Beatty Construction is re-cladding the two towers of Guy’s Hospital.
But this is no simple re-cladding job. The two 142.6m tall, 34-storey towers constitute the tallest hospital buildings in the world and inside them is a fully functioning hospital.
“Sequencing and logistics on this job is absolutely key. Now we’ve built the causeway, we go about our business without anyone knowing we’re there”
Chris Miller, Balfour Beatty
The work is not just a cosmetic makeover. As long ago as early 2000, client Guy’s & St Thomas’s NHS Foundation Trusts recognised that concrete was spalling from the towers and posing a serious risk to passers-by below (see box). A patch and mend job was ruled out as unsustainable in the long term, and complete reconstruction was not an option. Apart from the estimated £500M cost, there was the question of where the hospital’s functions could be temporarily rehoused, explains Chris Moriarty-Baker, programme manager for Guy’s estate manager Essentia.
“The trust quickly concluded that it wasn’t feasible to knock it down and start again. It was never going to be realistic to finance another scheme or deal with the disruption,” he notes. “So it had to look at what could be done to deal with the spalling.”
One of the unique features of the buildings was the use of lightweight concrete for all the structural elements, using lytag aggregate in place of traditional stone aggregate.
This greatly reduced the selfweight of the structures and improved their fire resistance. However, over time, the condition of the concrete has deteriorated and in particularly large areas of the Communications Tower, external walls have spalled, exposing the steel reinforcement to further environmental corrosion.
The User Tower concrete is generally in good condition, however, there are signs of localised spalling and reinforcement corrosion of the balcony slabs in some areas.
This general decline in the condition of the concrete, and the requirement to modernise the aesthetics of the two towers, has driven the need for the repair and re-cladding works.
A six-month feasibility study was commissioned to work out what could be done. Architect Penoyre & Prasad came up with a range of options from do minimum upwards. The cash-conscious NHS trust opted for the “quite minimum” option: re-cladding in its entirety the Communications Tower, which contains lifts and stairs, and adopting a half-and-half approach to the main User Tower - cleaning and repairing concrete on its suspended, precast concrete balconies and overcladding the life-expired double glazed windows.
And Penoyre & Prasad associate Neil Allfrey is not that disappointed that his more radical ideas were rejected.
“The building is what it is,” he says. “It’s a product of late 1960s design and is very much of its time. And while a lot of people hate it, when we started to explore it we found it has interesting touches; the designers of the time were exploring their material with exposed aggregate facades.
“Once the overclad is completed we’ve then got to go in and remove the existing windows in the main tower, in a live environment. The cladding is only part of it”
Chris Miller, Balfour Beatty
“So what we tried to do is play on its strength; capitalise on the horizontality of the main tower and its concrete balconies and play with the Communications Tower,” he explains.
But even this do minimum option is far from a minor refurb; a massive 8,000m2 of aluminium cladding panels are being installed in all elevations of the Communications Tower and a further 5,300m2 of glazing units are being installed in all elevations of the User Tower. In total 34,500m2 of concrete is being cleaned and extensive concrete repairs carried out.
The two Guy’s Hospital Towers are connected at each level by glazed walkways.
Constructed between 1971 and 1974, they are the tallest hospital buildings in the world at 122m and 142.6m tall. The 122m tall User Tower has the largest footprint. It houses most of the hospital’s wards and Kings College’s research laboratories. One of the most noticeable external features of the tower are balconies around the building at each level.
Structurally, the frame of this tower consists of external columns and a centralised core running the full length of the 18m long building in the north-south direction.
The Communications Tower houses the lifts, services and offices for the hospital. With its lift shafts and floor openings, this building has been constructed as an insitu concrete tube with a series of internal core walls and an outer structural insitu concrete skin. The noticeable feature of theCommunications Tower is the profiled outer surface formed in the external concrete walls using a reusable metal shuttering system.
The original plan was for the Communications Tower to form the central core for a much bigger hospital complex linking to up to three more user towers. These were never built, yet the building remains curiously fit for purpose; the User Tower, because it is largely free from services such as lifts and stairs, has and a very flexible floorplate, which suits the hospital as its function continues to evolve.
But the real challenge lies in the access constraints. “There is only one place it touches the ground,” notes Allfrey.
The height of the buildings means that cleaning, repair and cladding works are being carried out using mast climbers. But the close proximity of neighbouring buildings around the tower called for extensive temporary works in the form of an elevated gantry around the building, known as “the causeway”.
A 3.7m wide platform constructed around the building’s eighth and 11th floors, supported by cantilevered structural steel gallows brackets, acts as the base level for the gantry’s mast climbers and provides an area for the movement of materials around the site.
Balfour Beatty’s temporary works solution allows all access for the external works to be provided without the need to interrupt the internal operations of the hospital. “This not only allows us to transform the two towers without disturbing the building occupants, but also offers more effective management of the construction works,” explains Balfour Beatty chief engineer Shiraz Dudhia.
“Sequencing and logistics on this job is absolutely key,” agrees Balfour Beatty operations director Chris Miller. “Now we’ve built the causeway, we go about our business without anyone knowing we’re there.”
The structural steel gallows brackets are an engineering marvel. These right angled brackets with angled supports, have a cantilever span up to 4.4m and are fixed back to the concrete columns. They can support up to 17t of imposed load.
Harsco is providing the mast climbers that run vertically up and down each elevation of the towers. They are to be used for cleaning the user tower and re-cladding the Communications Tower. The mast climbers are, in turn, supported by Harsco’s scaffold platform that spans between the structural steelwork gallows brackets that cantilever above the neighbouring buildings.
These brackets, manufactured and fitted by PAD Contracts, were fixed to the User Tower columns using the balconies as access platforms.
The Communications Tower was a different kettle of fish. With no balconies to use, Harsco erected tied scaffold access platforms up to the eighth and 11th floor levels, where the brackets were to be fitted.
A glass roof to one of the hospital’s atriums is located adjacent to the north side of the Communications Tower and in this instance Harsco designed and erected a “bolt-on” scaffold that was fixed to the external wall and included additional ledger bracing and anchor fixings, to transfer the shear load back to the walls. These access scaffolds allowed safe installation of the Communications Tower gallows brackets, as well as being used for ongoing access for inspections throughout the works.
But the real challenge with the Communications Tower, notes Balfour Beatty temporary works engineer Dylan Wright, was right at the top where, bizarrely, a lecture theatre with a raking seating structure cantilevers out from the main structure. This area represents a significant challenge in providing safe access for the concrete repairs and re-cladding works.
Client: Guy’s and St Thomas’ NHS Trust
Contractor: Balfour Beatty
Subcontractors: Permasteelisa (facades), PAD (structural steelwork), Harsco (access and scaffold), Kafften (concrete repairs and cleaning)
Here, Harsco has developed a hanging scaffold that drops down the cantilevered elevation of the lecture theatre and supports a suspended access platform to the underside of the raking concrete soffit that is also to be over-clad by cladding contractor Permasteelisa.
It’s a cunning solution, but still one that demands a good head for heights, particularly for the scaffolders erecting the cantilever. “Essentially, you were going down a trapeze, so making sure the scaffolders were in the right mindset was very important,” says Balfour Beatty design manager and temporary works co-ordinator Jamie Lewis. “They couldn’t be too timid with heights.”
Neither can Balfour Beatty and its subcontractors, who are now around half way through the job. So far it’s going well, aided by good planning.
“The challenge of a refurb is that you can take nothing for granted,” explains Miller.
“Everything is bespoke on this job,” agrees Permasteelisa project manager Massimo Berti. “Every day we have some small problem and need to adapt, either with the fixing or the material we are fixing to; every week is different.”
To combat this, multiple solutions have been pre-planned.
“We wanted at least three approved solutions to any given problem so could immediately move to option B if there was a problem and then on to option C if that didn’t work,” says Miller.
It’s a strategy that is paying off.
“We’re a year in and there is one year to go,” says Balfour Beatty project manager John McCallion. “We’re on programme.”
That programme will see the external cladding work complete early this year, before work then goes indoors - opening up a new set of challenges. “Once the overclad is completed we’ve then got to go in and remove the existing windows in the main tower, in a live environment,” says Miller. “The cladding is only part of it.”
Time will tell if the internal work goes as smoothly as the external. But the omens are good - the client is happy.
Gallows Brackets explained
Harsco’s elevated scaffold platform is supported by structural steel gallows brackets designed by Balfour Beatty chief engineer Shiraz Dudhia’s six-man team in Balfour Beatty’s Temporary Works department.
The cantilevered brackets span up to 4.4m from the concrete structure, and are designed to support up to 17t of load. The user tower brackets are the longest spanning brackets, spanning an extra 900mm beyond the balconies.
The longer span incorporates two diagonal struts to minimise bending moments in the top chord and limit deflection. Furthermore, the horizontal top chord of the brackets are dropped 1m below the floor slab to avoid clashing with the precast balcony panels, so a vertical steel post and knee brace is included to ensure that the couple-action loads are applied as close to the floor slab as possible.
The Communications Tower gallows brackets have a total cantilever span of 3.76m and without the obstruction of the balconies, can be fixed to the wall at the floor slab levels, achieving greater efficiency and removing the need for the vertical steel posts. Further rationalisation of the Communications Tower brackets were made through a detailed review of the stresses and deflection of the top chord under various loading conditions. This resulted in a single diagonal strut being included, achieving a saving in materials and removing complexity in the fabrication process.
Structural steel gallows brackets were expected to be fitted to each concrete column of the user tower and at regular centres along the Communications Tower elevations, allowing efficient design of the steel brackets and minimising the concentrated lateral “push/pull” loads onto the tower structures.
However, a detailed review of the original structural engineering drawings supplied during the contract validation period threatened to scupper the plan. “They highlighted unexpected floor construction of the towers that made our initial proposal impossible,” explains Dudhia.
The floor construction of the User Tower is typically a concrete composite floor made up of a thin concrete slab spanning onto pre-cast concrete I-beams at 1m centres. The precast beams span up to 10m between the insitu concrete frame along the edge of the tower and the central core structure. For robustness and to meet disproportionate collapse requirements, the frame is tied together with solid concrete beamstrips that span between the core and alternate columns.
The columns without solid beam-strips were checked and deemed unsuitable to resist large lateral loads, so the total number of gallows brackets that could be fixed to the User Tower was halved, effectively doubling the loads at the allowable locations.
A similar arrangement was used to construct the Communications tower, with the floors typically being composite slabs and precast hollow block floors for the short spans between the lift shafts. Again, gallows brackets that were to be fixed directly to the external face of the Communications Tower wall had to be limited to locations of return walls and insitu beam-strips to provide the in-plane strength and stiffness to support the gallows brackets.
The impact of having fewer brackets is such that the steelwork for each one increased in size and weight, and the fixings into the structure had to resist significantly higher loads. Balfour Beatty’s temporary works engineers held a number of design workshops with Hilti to develop concrete anchor designs that could resist the increased tensile and shear loads, and effectively mobilise the strength of the concrete frame. This was particularly important for the user tower brackets, for which the loads are significantly higher and with anchors installed into the 610mm wide columns, edge distance effects reduce the anchor capacity.
Hilti’s HDA undercutting anchors were selected for the user tower brackets. Unlike most mechanical anchors that develop their strength from a tensile cone within the concrete, the undercut anchors use the compressive strength of the concrete, pushing the anchor capacity closer to the failure load of the steel. The reduced span and closer centres of the Communications Tower brackets meant that resin anchors that are more sympathetic to the integrity of the spalling concrete could be specified.
With limited access options and without the assistance of cranes, all structural steelwork had to be manhandled into position so size and weight was critical. The brackets were therefore designed as a kitusing channel sections weighing no greater than 120kg that could be bolted together on site to form the completed brackets.
Furthermore, key to reducing the size and weight of the steel sections was the ability to achieve effective lateral restraint to the brackets, in particular the diagonal compression chords. To achieve this efficiency, Balfour Beatty worked with Harsco to design the scaffold platform to provide lateral restraint to the gallows brackets. Each of the eight scaffold beams that spans over the brackets is cross-braced in plan and is connected to the top and bottom flanges with four Gravlock couplers. The plan-braced beams effectively form a diaphragm that locks the top chords of the gallows brackets to provide stability to global buckling effects.